WASHINGTON — Sitting on top of a modified Boeing 747 jet, the shuttle Discovery made a sentimental journey on Tuesday, visiting its old haunts in Florida as well as its new environs around the nation's capital. And thousands thronged to see it pass.

Tuesday's flight began just after dawn at NASA's Kennedy Space Center in Florida and ended at midday at Washington Dulles International Airport, adjacent to the Smithsonian National Air and Space Museum's Steven F. Udvar-Hazy Center in Virginia.

Over the next two days, the world's most traveled space plane will be lifted off its perch on NASA's Shuttle Carrier Aircraft and readied for Thursday's official handover to the Smithsonian.

Although Discovery hasn't flown in space since its final mission to the International Space Station, more than a year ago, the ferry flight was even more definitive as a signal that a 30-year era in spaceflight was finally going into the history books.

"It's kind of bittersweet," said Henry Taylor, one of the flight engineers on the Shuttle Carrier Aircraft. "This is the last flight, but it's great to have it here at the museum for people to see."

NASA's deputy administrator, Lori Garver, put a positive spin on the proceedings: "We're very proud," she told me. "I like to equate it to your child going off to college."

In a flood of social-media updates, witnesses to the final flight wondered whether Discovery's retirement also signaled that America's space aspirations were fading. Garver would have none of that.

"To those who say our best days of space exploration are behind us, I simply must disagree," she told dignitaries, journalists and other guests assembled on the runway alongside the shuttle-jet combo. She pointed to NASA's future plans to go beyond Earth orbit, and eventually to a near-Earth asteroid and beyond.

Nevertheless, Tuesday's events were clearly more of a time to look back than to look ahead. Among those in the crowd was Ron Bledsoe, a 38-year-old resident of Manassas, Va., who was one of 50 people invited to the landing as part of a promotion for the Dulles airport's 50th anniversary.

"I feel like a 10-year-old kid again," Bledsoe told me.

He wasn't alone. Thousands turned out to cheer the shuttle's flight — first at Kennedy Space Center, where about 2,000 shuttle program veterans paid their last tribute to Discovery. Still more watched the skies from Florida's Space Coast, where Discovery lingered before heading up the East Coast.

In Washington, the National Mall filled with onlookers who watched the shuttle-jet combo fly over the U.S. Capitol, the White House and other historic monuments at an altitude of 1,500 feet. "Oh my God, look at that," Terri Jacobsen of Bethesda, Md., told The Associated Press when she first spotted the double-decker craft. "That thing is mammoth."

"It was pretty amazing," her 12-year-old son, Riley, said later. "Pretty freaking crazy. It looked like it was inflated."

bThe Smithsonian's curator for shuttle artifacts, Valerie Neal, was all smiles when Discovery and its carrier airplane touched down at Dulles, a little more than four hours after its Florida takeoff. Discovery will take the place that had been held by the shuttle Enterprise, a prototype shuttle that was handed over to the Smithsonian in 1985.

On Thursday, Discovery and Enterprise will be displayed together outside the Udvar-Hazy Center. In an interview, Neal mused over the place that the shuttles will hold in history for future generations.

"I'd love to be here 100 years from now," she said. "People might say, 'Well, isn't that quaint?'"

There's nothing quaint about Discovery's 28-year history as a spaceship: The orbiter has flown 39 missions, more than any other single spacecraft. It has logged more than 148 million miles of travel, over more than a year's worth of days in space.

Discovery's list of achievements includes delivering the Hubble Space Telescope to orbit, carrying the first Russian cosmonaut to launch on a U.S. spaceship, performing the first rendezvous with Russia's Mir space station (with the first female shuttle pilot in the cockpit), returning Mercury astronaut John Glenn to orbit, and bringing shuttle flights back to life after the Challenger explosion in 1986 and the Columbia tragedy in 2003.

Three more shuttle shifts remain: Enterprise will be ferried to New York City as early as next week and will eventually go on display at the Intrepid Sea, Air and Space Museum. The shuttle Endeavour will be flown to the California Science Center in Los Angeles this fall. Atlantis will have the shortest journey of all: It will be towed from NASA's Kennedy Space Center to the nearby visitor center in Florida.

With the shuttles in retirement, private U.S. companies hope to pick up the slack, beginning with space station cargo and then, hopefully, astronauts. The first commercial cargo run, by California-based SpaceX, is set to launch from Florida on April 30.

For at least the next three to five years — until commercial passenger craft are available in the United States — NASA astronauts will have to hitch multimillion-dollar rides on Russian Soyuz capsules to get to the International Space Station.

Video: Discovery shuttle lands in D.C.

Closed captioning of: Discovery shuttle lands in D.C.

>>>there was something special in the skies today from florida north to washington along the
east coast
. it stopped traffic and it brought people out their homes and offices. it was a
final farewell
for the
space shuttle
"
discovery
," latched to the top of a 747, flying around as if to waive good-bye on its way to retirement at the smithsonian. turns out "
discovery
" had one more mission left in it, as
nbc
's
anne thompson
reports.

>> reporter: "
discovery
's" sentimental journey to washington began early this morning, taking off from florida's
kennedy space center
on the back of a 747. flying over its launch pads and beaches where generations watched its liftoffs starting in
1984
.

>>this is the biggest foundation of knowledge that we have built. we're 50 years ahead where we would have been had we not a
space program
.

>> reporter: "
discovery
's" career marked with historic moments.
eileen collins
flew "
discovery
."
john glenn
the former astronaut and senator, the oldest person to fly in space did so on "
discovery
." it helped build the
international space station
and deployed the
hubble telescope
. and in times of tragedy, after the disasters of the challenger and columbia, "
discovery
" led america back into space. it was fitting today that "
discovery
's" victory lap brought together
christa mcauliffe
's belief that space is for everybody.

>>is that the coolest thing you've ever seen? yeah.

>>ever?

>>yeah.

>> reporter: people looked up in awe as "
discovery
" flew by the monument, the capitol and the
white house
. a sight so spectacular it stopped traffic on the ground. drew teachers and students out of classrooms, even pilots in the air wanted to see.

>>a 747 with a shuttle on the back is landing -- he's about 15 miles out now from the smithsonian.

>> reporter: "
discovery
" landed for the final time at
dulles airport
, a bittersweet moment for janet cavandi.

>>we wanted it to fly for a long time before it touched down.

>> reporter: now "
discovery
" will have an earthbound role, to inspire a new generation of americans for the exciting adventure of space.
anne thompson
,
nbc
news, new

Discovery climbs into orbit after launch on July 26, 2005, at the Kennedy Space Center in Florida. The seven-person international crew departed for a 12-day mission to the International Space Station.
(Bob Pearson / AFP / Getty Images)
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First lady Laura Bush applauds during the liftoff of Discovery on July 26, 2005, in the stands at the Kennedy Space Center's Banana Creek viewing site in Cape Canaveral, Fla.
(NASA via Reuters)
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Tourists cheer at the Kennedy Space Center Visitor Complex at the sight of the space shuttle Discovery lifting off on Tuesday, July 26, 2005.
(Scott Audette / AP)
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Astronaut Stephen Robinson, anchored to a foot restraint on the International Space Station's Canadarm2, participates in the mission's third spacewalk on Aug. 3, 2005.
(NASA via AP)
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Chigasaki residents celebrate the U.S. space shuttle Discovery's safe return to Edwards Air Force Base in California's Mojave Desert on Aug. 9, 2005, at Hamasuka Junior High School in Chigasaki, west of Tokyo. Chigasaki is the hometown of Japanese astronaut Soichi Noguchi, one of seven crew members.
(Koji Sasahara / AP)
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Discovery's nose landing gear was photographed by the crew on the International Space Station on July 28, 2005, as it did a pitch maneuver for tile inspection before docking. The crew of Discovery moved onboard the space station after carrying out new shuttle damage checks as ordered by NASA after a suspension of flights over safety concerns. NASA halted the program again after Discovery's return because of debris that fell off during its launch.
(Nasa Photo / AFP / Getty Images)
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The Discovery hitches a ride from California to the Kennedy Space Center in Florida on a special 747 carrier aircraft on Aug. 19, 2005. The shuttle landed in California on Aug. 9 as weather conditions prevented it from landing at Cape Canaveral, Fla., as originally planned. Discovery's mission was the first flight for the shuttle since Columbia broke upon re-entry in February 2003.
(Lori Losey / NASA)
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Astronaut Piers J. Sellers, STS-121 mission specialist, wears a training version of the Extravehicular Mobility Unit spacesuit while participating in a simulation at Johnson Space Center. The RMS has a 50-foot boom extension, called the Orbiter Boom Sensor System, attached. It would be used to reach beneath the orbiter to access tiles. Lora Bailey, right, manager of JSC Engineering Tile Repair, assisted Sellers.
(NASA)
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Workers observe the Discovery before it begins its six-hour trek from the vehicle assembly building to Launch Pad 39B at the Kennedy Space Center in Cape Canaveral, Fla., on May 19, 2006.
(John Raoux / AP)
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In the White Room on Launch Pad 39B, STS-121 mission specialist Thomas Reiter of Germany, representing the European Space Agency, gets final adjustments made to his launch suit before entering Discovery at Kennedy Space Center in Cape Canaveral, Fla., on June 15, 2006.
(NASA via EPA)
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Astronauts Thomas Reiter of Germany and Piers J. Sellers and Stephanie D. Wison of the U.S., all STS-121 mission specialists, train in advance of their launch on July 1, 2006.
(NASA via AP)
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NASA spokesman Bruce Buckingham listens as John Shannon, NASA's deputy space shuttle program manager, points to a location on a model of the external fuel tank where a piece of foam insulation broke away from Discovery. The mishap was explained during a news conference on July 3, 2006, at the Kennedy Space Center in Cape Canaveral, Fla.
(Mark Wilson / Getty Images)
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The Discovery lifts off on another mission from the Kennedy Space Center in Cape Canaveral, Fla., on July 4, 2006.
(NASA via Reuters)
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Self-portrait, taken by astronaut Michael Fossum on July 8, 2006, during a spacewalk while the Discovery orbiter was docked with the International Space Station. Turning his camera to snap a picture of his own helmet visor, he also recorded the reflection of his fellow mission specialist, Piers J. Sellers, near center of picture, and one of the space station's gold-tinted solar power arrays arcing across the top. The horizon of Earth is in background.
(Michael Fossum / NASA)
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The nose of Discovery and part of the underside is seen over Earth on July 6, 2006. NASA engineers examined detailed pictures of the space shuttle's heat shield a day before two astronauts were to embark on the most disorienting task of their 13-day mission: a wobbly spacewalk.
(NASA via AP)
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The sun illuminates the Earth's atmosphere during a sunrise, seen from the Discovery after departure from the International Space Station on Aug. 6, 2005. A portion of the shuttle's aft cargo bay, its vertical stabilizer and orbital maneuvering system pods are seen in the foreground.
(AP)
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A lightning bolt crackles down in the distance on Aug. 14, 2006, during preparations at Edwards Air Force Base in California to return the shuttle Discovery to its Florida home base. The gantry-style structure surrounding Discovery is used to mount the shuttle atop a modified Boeing 747 jet for a cross-country piggyback flight.
(Tom Tschida / NASA via Getty Images)
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Discovery lands at the Kennedy Space Center's Shuttle Landing Facility in Cape Canaveral, Fla., on June 14, 2008. The shuttle completed a 14-day mission to the International Space Station, where it delivered the Japanese Kibo module. The STS-124 mission also included three spacewalks.
(Justin Dernier / EPA)
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Discovery approaches the International Space Station during rendezvous and docking operations on June 2, 2008. The second component of the Japan Aerospace Exploration Agency's Kibo laboratory, the Japanese Pressurized Module, is visible in Discovery's cargo bay.
(AP)
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A woman kicks back as she watches from Titusville, Fla., as the space shuttle Discovery launches from Pad 39A at the Kennedy Space Center in Cape Canaveral on May 31, 2008.
(Eric Thayer / Reuters)
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With Earth as a backdrop, Discovery approaches the International Space Station during STS-133 rendezvous and docking operations on Feb. 26, 2011. Discovery, on its 39th and final flight, carried up the Italian-built Permanent Multipurpose Module, Express Logistics Carrier 4 and Robonaut 2, the first humanoid robot in space.
(NASA via Getty Images)
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Editor's note:
This image contains graphic content that some viewers may find disturbing.

In a highly-anticipated speech to Congress Tuesday, Israeli Prime Minister Benjamin Netanyahu argued that a potential nuclear deal being negotiated by major powers including the United States "paves Iran's path to the bomb."